Use nested namespace declarations

[mikomikotaishi]

The library currently uses repeated namespace declarations to nested namespaces. As of C++17, nested namespace declarations were introduced and allow concisely a single declaration for nested namespaces:

// currently...
namespace Poco {
namespace Net {
namespace ODBC { 
    // ...
}}} // namespace Poco::Data::ODBC

// proposed new-style (C++17 onward)
namespace Poco::Net::ODBC {
    // ...
} // namespace Poco::Data::ODBC

As this syntax was introduced in C++17, and the library is C++20, there should not be any code breaking in such a change.

This would probably be difficult to do by hand, so perhaps a Python script could be used to do this quickly.